1. Field of the Invention
The present invention relates to a heat-sensitive recording sheet having high stability of background brightness and of developed image against oily substances such as hair oil, oil, fat, etc.
2. Prior Art
A heat-sensitive recording sheet that utilizes a thermal color-forming reaction occurring between colorless or pale-colored chromogenic dyestuff and phenolic material, or organic acid is disclosed, for example, in the Japanese Patent Publication Nos. 4160/1968 and 14039/1970 and in the Japanese Laid-Open Patent Application No. 27736/1973, and is now widely applied for practical use.
In general, a heat-sensitive recording sheet is produced by applying the sheet surface with the coating which is prepared by individually grinding and dispersing colorless chromogenic dyestuff and color-developing material such as phenolic substance into fine particles, mixing the resultant dispersions with each other and then adding thereto binder, filler, sensitizer, slipping agent and other auxiliaries. When this sheet is heated, the coating undergoes instantaneously a chemical reaction which forms a color. In this case, various bright colors can be advantageously formed depending upon selection of specific colorless chromogenic dyestuff.
These heat-sensitive recording sheets have now been found in a wide range of applications, including medical or industrial measurement recording instruments, terminal printers of computer and information communication systems, facsimile equipments, printers of electronic calculators, automatic ticket vending machines and so on. And the requirements for improving following basic properties are increasing: (1) high image density and clear image, (2) no accumulated residues, no sticking etc., (3) superior recording aptitude and less decline of brightness in the time elapsed, and so on.
The applicant has already disclosed in Japanese Patent Laid-Open No. 144193/1981 and Japanese Patent Application No. 55974/1982 that a heat-sensitive recording sheet comprising a mono-phenolic 4-hydroxyphenyl compound such as 4-hydroxybenzoic acid ester and 4-hydroxyphthalic acid diester as a color developer in combination with a fluoran type dyestuff is excellent in the above-mentioned fundamental quality.
Heat-sensitive recording sheets are inevitably in contact with human hands in view of the function thereof as information recording sheets. Since oily substances such as conventionally used hair cosmetics or oil and fats contained in sweats appearing on skins often adhere to the hands and fingers, the heat-sensitive recording sheets may frequently be contaminated by these oily substances. By the way, since heat-sensitive recording sheets are not generally so stable against these oily substances, image density in the contaminated area may be reduced or sometimes be eliminated utterly, or discoloration occurs in the contaminated white area. Although the reasons for the above phenomena have not yet been cleared completely at present, it may be considered that the oily substances partially dissolve or instabilize the chromophoric layer or the chromophoric reaction products therein formed between fine particles of a colorless basic dyestuff and an organic color-developing agent.
As mentioned above, it has been found that although the heat-sensitive recording sheet comprising 4-hydroxy-benzoic acid ester or 4-hydroxyphthalic acid diester as a color-developing agent is excellent in fundamental qualities, its stability against oily substances is somewhat inferior to that of bisphenol type color-developing agent customarily used so far.